Frequency selective transmission network



April 18, 1944. s. Y. WHITE 'FREQUENCY sELN'cTvE TRANSMISSION NETWORK Filed April l. 1942 2 Sheets-Sheet 1 ATTORNEYS j April 184, 1944. s. Y. WHITE 21,346,804

y FREQUENCY SELECTIVE TRNSMISSION NETWORK i Filed April 1, 1942 2 sheets-sneer 2l AT TORNE YS Patented Apr. 18, 1944 FREQUENCY SELECTIVE TRANSMISSION NETWORK Sidney Y. White, Wilmette, Ill., assignor to Victor S. Johnson, Chicago, Ill.; ,Alex Thomson administrator of said Victor S. Johnson,'de-

l ceased Application April 1, 1942, .SerialNm 437,140

(Cl. Z50-20) (i Claims.

It is known that a frequency responsive electrical transmission network which provides full output response to a predetermined tone :frequency of normal amplitude and which provides an output response of much lesser value for a fre quency which differs from the predetermined .to-ne frequency by a rather small amount as, for example, l0%, may be constructed by connecting a series of highly selective resonant circuits in cascade. Each of such highly selective resonant cirl cuits ordinarily has a high ratio of inductance to resistance (high Q), this ratio having a value of the order of from 100 to 160 in the selective circuits of .the .usual radio receiver. Such a frequency selective network which .is responsive `to audio frequencies .of the order of 5000 to 7.000 cycles necessarily has bulky coils so that it takes up a large amount of space and runs into considerable weight and :is .expensive to construct. It is accordingly :an object of the .present .invention to provide a frequency selective transmission network which is highly selective Aas vto different f-requenciesyet is at the same time of small 'weight and size and inexpensive to construct.

It is known that such high Q circuits have a small decrement .and it follows that a large number of voltage impulses of the resonant tone frequency must be .applied before the current in the rst circuit `of :the series is .built up to its full value and an .additional time :must elapse before the current inthe .other circuits :in succession .reaches its maximum-value. In certain applications speed of operation of a frequency .selective network lis of .great importance and highly advantageous vand it is one of the purposes .of :the present .invention to provide a frequency selective .network comprising one or a few resonant circuits `in which the ratio of the inductance to .the resistance has a low value, a value of the order of :from `18 to `2-5 having been found suitable. :Such circuits .have a high decrement and conversely with-in .a ifew cycles after the application .of the lresonant tone frequency thereto, :the full output voltage is :built up in the .last circuit ofthe series, .In the rillustrated embodiment of v:the invention, :in which a pair of coupled circuits are resonant to v"7000 y.cycles and :in which `the ratio of the inductance to the resistance of each circuit has a yvalue .of-ap.- proximately 20, only about 20 cycles .of Ithe energizing voltage :has to be .applied before .the full output voltage .is ldeveloped across .the circuits. This corresponds .to a :time of 3 n'xillisecor-tds:and it will 'be .understood ythat :in .other linstallations `responsive to .higher frequencies, vthe full A'response Vmay be built up within astill ,shorter time `:interas val, due to the greater number of cycles Ain a given unit Iof time. In the illustrated embodiment of the invention, two resonant circuits connected in cascade .are ,shown atlhough it will be understood that in certain installations .a single resonant .circuit may be found to provide sufficient selectivity and in other installations where a still greater degree of selectivity is desired three lor more such resonant circuits may be connected in cascade,

Any resonant device can be driven to any am plitude at any frequency by the application oi sufficient power. It is an important feature of the present `invention that the amount of power supplied to the resonant circuit or circuit-s is limited to a predetermined value which ,is insufli.- cient to develop .any output voltage across the output terminals of the frequency ,selective network for any .applied frequency which differs from the resonant .frequency lby l0 per .cent or more. For this purpose a Alimiter' tube is used, the signal frequency being applied to its grid and its plate circuit being coupled to lthe resonant circuit to supply a limited amount of energy thereto. In the illustrated embodiment of the invention, this energy is sufdcient for -a ,signal of the resonant frequency and normal strength to develop a peak output voltage across the resonant circuit of .approximately 20 volts. However, fora signalfrequency which differs .from theresonant frequency by 10 per cent, .the energy supplied by vthe limiter tube for a signal of :maximum possible -strength is only sufficient to .develop a voltage of .substantially 9 volts .across the resonant circuit.

it is a further object of the vinvention to lprevent ysuch a voltage generated by .an undesired frequency `from causingfany output voltage whatsoever to appear across the 'output terminals of the frequency .selectivenetwork This is secured by providing `as part of the frequency selective network .a device .associated with `the resonant circuit or circuits which is non-responsive to input-voltages below a predetermined level. This device is so arranged that for any .usable input voltage .of the resonant frequency a substantial vdirect :current routput voltage is developed across the output terminals :of the transmission vnetwork while for 4an input voltage of any magnitude, no matter how large, having a Ifrequency differing :from `the .resonant frequency byV .substantially 10%, no .output voltage whatever is developed across Ioutput `terminals of ythe network. Stated in other terms, the ratio between theoutpu't voltage for the desired frequency and that for an undesired frequency is infinitely large, corresponding to an infinite degree of selectivity.

If a very large transient voltage were applied to the resonant circuits of a frequency selective network, the resonant circuit or circuits thereof may be caused to oscillate at their resonant frequency by shock excitation and generate a substantial voltage across the output terminals of the network.

In' applicants co-pending application Serial No. 321,378, filed February 29, 1940, now Patent No. 2,283,523, dated May 19, 1942, of which this application is a continuation in part, the frequency selective network is shown as forming part of a radio receiver which automatically scans the frequency spectrum and locks on a received carrier modulated by the tone frequency to which the frequency selective network is responsive. The invention however is not limited to such use but has a wide field of application and is adapted for use in any electrical system in which different devices are provided each designed to be operated by a different tone frequency such, for example, as multiplex facsimile, teletype and telegraph'systems.

For a better understanding of the invention reference is made to the following description taken in connection with the accompanying drawings, in which:

Fig. 1 is a schematic circuit diagram of an electrical transmission network embodying the invention;

Fig. 2 shows the response curves across one of the circuits of Fig. l for different applied frequencies;

Fig. 3 is a sectional view showing the construction of a transformer unit embodying the invention; and

Figs. 4 and 5 are schematic views illustrating certain mechanical arrangements explanatory of the operation of the invention.

Referring to Fig. 1, the invention is shown as comprising a limiter tube VT'I to whose grid a source of voltage I of variable tone frequency -is connectedf the usual grid biasing resistor II being connected in the cathode circuit and the plate of the tube being resistance coupled to the resonant circuit TC3 by the resistor RI'I and condenser CI. The circuit TCS comprises the primary transformer winding or coil I2 and a trimmer condenser I3 which is adjustable to vtune the circuit to the desired tone frequency, which may be in the sub-audio, audio or superaudio range of frequencies. The coil I2 is optimum coupled to the secondary winding I4 of the resonant circuit TC4 so as to transfer the maximum energy to this circuit at the resonant frequency. A trimmer condenser I5 is provided for tuning the circuit TC4 to the tone frequency. In order to greatly increase the inductance of the resonant circuits, each of the coils I2 and I4 is provided with a compressed powdered iron core I6 and I'I, the size of the iron particles winch compose these cores being suitable for the operating frequency. The lower terminal of the circuit TC4 .is connected to the cathode IS of the diode rectifier tube VTS through the load resistor RII and a volt bias battery 20 while the upper terminal of the circuit is connected to the plate I8 of the diode. The rectified voltage developed across RII is filtered bythe lter circuit comprising resistor RIZ and condenser C8 and appears .across the output terminals 2I oi the transmission network as a direct current voltage Eo.

`voltage thereto. these circuits are designed to provide a low ratio .of inductance to resistance (Q) as, for example,

Referring to Fig. 2, the full line curve A indicates the peak volts developed across the resonant circuit TC4 for different tone frequencies of the normal amplitude of 5 volts generated by the source I Il, the circuits TC3 and TC4 being resonant at 7 kc. At this frequency it will be observed that a maximum peak voltage of 20 volts is developed acrossV the circuit TC4, but since the diode VT9 has an initial negative bias voltage of -10 volts, the useful net output voltage Eo appearing across the terminals 2l has a value of 10 volts, as indicated on the scale at the right of Fig. 2. It Will also be observed that at frequencies of 6 kc. and 8 kc. only about 2 volts is developed across the circuit T04, which is lnsufficient to overcome the 10 volts negative bias of the diode VTS and therefore produces no output voltage Eo whatsoever across the output terminals 2E. The curve B in dash lines shows the peak voltage across the resonant circuit TC4 when the maximum strength of signal is applied to the grid of tube VTI and when the current in its plate circuit has been fully modulated and beyond which no further increase in the Power delivered by the plate circuit is possible by reason of the limiting action of the tube. In the illustrated embodiment of the invention, it is found that beyond a voltage of 15 volts applied to the grid of the limiter tube VT'I no further increase in modulation of its Plate current takes place so that the power supplied to the resonant primary circuit TG3 is definitely limited to a predetermined maximum value. From curve B it will be observed that for frequencies of 6 kc. and 8 kc. only about 6 volts is developed across the circuit T04' which, of course, is insucient to overcome the 10 volts bias of the diode and develop any output voltage Eo across the output terminals 2| of the transmission network. From an inspection of curve B it will also be observed that for applied frequencies which differ from the resonant frequency of 7 kc. by more than approximately 650 cycles, no direct current output voltage is developed across the output terminals ZI, so that as between such frequencies and the resonant frequency, the transmission network has infinite selectivity.

If current were supplied to the circuit TO3 from a source which was capable of supplying a large amount of power or energy thereto, it would be possible for large transient voltages of steep slope to energize the resonant circuits TG3-T04 by shock excitation and thus cause them to oscillate at full amplitude at the frequency of 7 kc. This action would cause an out` put voltage Eo to appear, even in the absence of the tone frequency to which the network is normally responsive. This action is prevented, however, by the use of the limiter tube VT'I whose plate circuit delivers insuflicient power to the circuit TC3 in response to the impression on its grid of an abrupt transient voltage of any amplitude to cause this circuit to oscillate strongly at its resonant frequency and thus generate an output voltage Eo across the terminals 2 I.

A feature of the invention is the construction of the resonant circuits TG3 and T04 in such a manner that they have a high decrement and become fully excited within a very short time after the application of the resonant frequency For this purpose the coils of a ratio of the order of from 18 to 26 and in the illustrated embodiment .of the invention, a Q of was found suitable. With such low -Q coils, it is feasible to develop the full output voltage at the terminals of the transmission network within 3 milliseconds after the application of the tone frequency to vits input terminals. This 'rapidity of response is highly important .in certain' applications of frequency selective networks and -it will be understood that by using circuits havingia somewhat lower Q, the full output response 'may be developed withina still Ashortertime .interval at the' same .frequency 'after the application of the energizing voltage.

The action of the limiting tube VTI and the biasedidiode VTS in connection f with the resonantfcircuits maybe understood more clearly upon consideration of the analogous :mechanical systems schematically shown in Figs; 4 andi. In Fig. 4 a pendulum 22 is shown supported for freeoscillation within a closed 'casing4 V23 provided at one end with an opening 24. `A rsmall weight is suspended on a string 2b so as to extend into the opening 24 and to `strike against the bob 21 `of the pendulum 22 when struck lightly by a hammer 28. vBy making the-bob 27| yrelatively small and of a light metal, :suchv as alume num, the pendulum 22 will .have a large decrement. Also by striking the 'weight .25 bythe hammer 2B repeatedly at the natural-period vof vibration of the pendulum 22, the pendulum will be 'set into oscillation at its natural period and only a small number of energy impulses as, 'for example, 20 by the weight V25, will be required to cause the pendulum to oscillate through its lfull range of movement. The bob 21 is adapted 4to engage a switch lever 29 which is normally .bi-

ased against a stationary stop '30 by va .spring 3|, so that as soon as the swing of pendulum v22 reaches its full amplitude the `switch 29 is closed establishing the flow of current through a local circuit including a .battery 32 yand bell 33. The pendulum 22 of small mass and `high damping corresponds to the low Q. circuits YTC3--1'IC4 :in that only .a small number of energizing impulses at the proper frequency are required .to 4cause the pendulum to vibrate fully .at Aits natural .period, 'which in vturn causes the bell 33 to respond.

In the arrangement of Fig. 5, the pendulum 34 is not enclosed and while it is of the same length vand has the same natural period as the pendulum 22, yet its bob is made of lead or other vheavy metal and has a mass many times that ofthe lbob 2l. This pendulum has a very much `lesser damping and decrement than the pendulum 22 and corresponds to a high Q circuit in that it must be struck a large number as, for example, 200` blows at its .natural period by the small hammer 23 before it lwill oscillate at its full amplitude and thus close the switch .29 and ycause the operation of the bell 33. YA high Q circuit, having a -smalldamping and decrement, requires the `application thereto of `a large number of small voltage impulses of its resonant frequency before it becomes fully excited. Should, however, the bob 35 be struck va single hard blow by a vmassive hammer, which would correspond `to the `full excitation of the circuit TG3 by a large transient voltage, the pendulum 34 will at once be set .into full oscillation at its natural period and immediately close the switch 29 and cause the operation of the bell 33. This impulse excitation of the pendulum '22 by the hammer 28 is prevented however by reason of the interposition between 'the hammer and the bob 2l of a `weight 25 of such small mass 'that it possesses insuilicient energy to move fthe bob any substantial distance by va single blow even if the weight 25 is struck hard by a large hammer. The arrangement of Fig. 4 thus permits the bell 33 to respond rather quickly to Vsmall impulses of the proper frequency applied -to weight 25 while at the same time causing no operation of the bell in response to a single shock impulse of the weight 25, however, large. The function performed by the weight 25 lcorresponds to that performed in the circuit arrangement of Fig. 1 by the limiter tube VT`I, the plate circuit ofthis tube supplying insufflcient energy to the resonant circuit TG3 to build up sufficient current in this circuit to cause any output voltage whatever to be developed across the terminals 2| even in response to the impression of a large transient voltage on thev grid circuit of tube VTT. The circuit arrangement is thus seen to be quickly responsive to the desired tone frequency and 'to be entirely non-responsive to frequencies which differ therefrom by 10% or more and also to large transient voltages.

It will be understood that any other known type o1 electrically operated device which requires the application of a voltage above `a 'predetermined minimum value lbefore it becomes energized may be substituted in place of the biased diode VTS as, for example, certain types of thyratron tubes or a triode detector having its grid biased beyond cut-olf.

A preferred mechanical arrangement of the circuits TG3-TG3 is shown in Fig. 3 in which a rectangular supporting base 35 of Bakelite is provided with a central aperture '31 and 5 project-ing prongs 38 (only two being shown) adapted to be plugged into a socket. A pair kof vertical brass supporting strips '39 have their lower ends bent over and secured to the base .3S by screws 4G, a shelf '4I being secured to the upper end of the strips S9 by the screws 52. A lower shelf d3 is supported on a pair of small brass brackets :li which are secured to the supporting strips 39 by screws 45. A pair of U-shaped brass `brackets i6 are secured to `the strips '39 A.by screws B7, these brackets `supporting a pair of spaced apart shelves 48, 49 which are secured thereto by screws 56--5L V'l'.'he coil supporting shelves 4i, 43, 48 and 4E are formed of suitable insulation material such as Bakelite The coil 'l2 with its powdered iron core i6 is held in position by being squeezed 'between the shelves 53 and 49 while the secondary coil i4 with its pow'- clered iron core l1 is held coaxial with the coil l2 by being squeezed between the upper shelves fil- 48. The trimmer condenser I3 'for they coil i2 is aiilXed to the under side ofshelf' 43 and is provided with an adjusting screw 52'which may be operated by a screw-driver inserted through the opening 31 for the purpose of adjusting the capacity of the condenser to tune the circuit TG3 to 'the tone frequency. The trimmer condenser "l5 for tuning vthe circuit TCfi to the tone vlfrequency is secured to the upper shelf "lil and is provided with an adjusting screw 53. The component 'parts of the resonant circuits 'are enclosed in a rectangular shaped metallic shield can 5A which is provided at its top with an open'- 'ing V55 through which the screw 53 projects slightly. The coils l2 and Ill are of layer wound copper wire with paper insulation between the layers and each coil is approximately 1 inch long and of "V8 inch diameter, the diameter of the iron cores being approximately inch. The size-df the aluminum shield can '54 is l1/2 x^2 x 4 inches and the weight oi the entire plug-in assembly is only 4 ounces. By means of the construction described, a number of plug-in units may be provided, each tuned to a different tone frequency so that the transmission network may be quickly made responsive. to any tone frequency by plugging in a unit resonant to that frequency.

'I'he plug-in construction described providesfa ready means for assembling and supporting low Q coils and for securing optimum coupling between the primary and secondary windings. As shown in the curves of Fig. 2, this arrangement has response curves which are relatively broad at their upper portions.

In a transmission network responsive to a fre quency of '7000 cycles, the following circuit constants were found suitable.

C1=250 mmf. RII-125,000 ohms C8=150 mmf. RII=100,000 ohms RIZ- 50,000 ohms Inductance of coils l2 and I4 from 130 mh. to

Q of coils l2 and I4 from 26 mh. to 18 mh.

Vacuum tubes of the following types are found suitable for use with the circuits described.

VT`|=oney section of a 6N? double triode VT9=double diode triode, 6R7

I have described what I believe to be the best embodiments of my invention. I do not wish, however, to be conned to the embodiments shown, but what I desire to cover by Letters Patent is set forth in the appended claims.

I claim:

1. A frequency selective transmission network comprising, in combination, a limiter tube, a resonant circuit having a low ratio of reactance to resistance and a detecting device connected in cascade and including parts which are unresponsively related below a predetermined threshold value of applied voltage, said parts having interacting and responsive relationship above said threshold value of applied voltage, whereby the ratio between the selectivity and the number of cycles required to build up to full amplitude in the output of said transmission network is mark.- edly increased over the ratio inherent in any resonant circuit alone.

2. A frequency selective transmission network comprising, in combination, at least one circuit resonant to a predetermined tone frequency, a limiter tube arranged to impress voltages of a range of frequencies including said tone frequency on said resonant circuit, a detecting device in cluding parts which are unresponsively related below a predetermined threshold value of applied voltage, said parts having -interacting and responsive relationship above said threshold value of applied voltage, a circuit connecting said detecting device to said resonant circuit, and a load impedance in the output circuit of said detecting device, said threshold voltage having such a value that upon the impression of a voltage of said tone frequency in excess of a predetermined minimum value on the input circuit of the limiter tubea direct current voltage of substantial amount is developed across said load impedance but upon the impression on the input circuit of the tube of a voltage of any magnitude having a frequency differing by at least 10 per cent from said tone frequency, no voltage is developed across said load impedance.

3. A frequency selective transmission network comprising, in combination, at least one circuit resonant to a predetermined tone frequency, a limiterY tube` arranged to impress voltages of a range of frequencies including said tone frequency on said circuit,`adetecting device, and a circuit comprising a source of steady'bias voltage and aload impedance connecting the terminals of the detectingdevice to said resonant circuit, said source of bias voltage having such a value that upon the impression of a voltage of saidV tone frequency in-excess of a predetermined minimum value-ion the input circuit of the limiter tube, a voltage of substantial value is developed across said load .impedance but upon the impression on the input circuitf the limiter tube of a voltage ofv'any magnitude having a frequency diieringiby at least 10 per cent Ifrom said tone frequency, no voltage is developed across said load impedance.

4. A frequencyl selective transmission network comprising, in combination, at least one` circuit resonant to a predetermined tone frequency, a.v

limiter tube arranged to impress voltages of a range of frequencies including said tone frequency on said resonant circuit, a detecting device con-` nected in cascade and including parts which are unresponsively related below a predetermined mim imum threshold value of applied voltage, said parts having interacting and responsive relationship above said threshold value of applied voltage, said device coupled to said resonant circuit and a load impedance in the output circuit of said detectingdevice, said threshold voltage having such a value that upon the impression of a voltage of any amplitude having a frequency diering by at least 10 per cent from said tone frequency no voltage is developed across said load impedance but upon the impression of a. small number ofv cycles of a voltage of said tone frequency in excess of a Vpredetermined minimum value on the input circuit of the limiter tube, a. direct voltage of substantial amount is developed across said load impedance.

5. A frequency selective transmission network comprising, in combination, at least one circuit resonant to a predetermined tone frequency, a limiter tube arranged to impress voltages of a range of frequencies including said tone frequency on said resonant circuit, a detecting device connected in cascade and including parts which 'are unresponsively related below a predetermined minimum threshold value of applied voltage, said parts having interacting and responsive relationship above said threshold value of applied voltage coupled to said resonant cir.- cuit, and a load impedance in the output circuit of said detecting device, said threshold voltage having such a value that upon the impression of a voltage of said tone frequency in excess of a predetermined minimum value on the input circuit of the'liimter tube a direct current voltage of substantial amount is developed across said load impedance but upon the impression of a single transient pulse of any voltage or slope on the input circuit of the tube, no voltage is developed across said load impedance.

6. A frequency selective transmission network comprising, in combination, a. driver tube having an output circuit, a detecting device having a load circuit including a source of steady bias voltage, and at least one circuit resonant to a predetermined tone frequency and arranged to couple the output circuit of said driver tube to said detect; ing device, said resonant circuit being so designed that the ratio of the reactance to the resistance thereof has such a low value that upon the excitation of the resonant circuit by current of the tone frequency in the output circuit of the driver tube, the current in the resonant circuit builds up to maximum value Within a few cycles thereby generating in the load circuit of the detecting device a voltage which is greater than that of the bias voltage source and causing a direct current to ow in said load circuit, the constants of said driver tube and its output circuit being so selected that for a current therein of a fre quency only slightly diierent than said tone frequency, the voltage built up in the resonant circuit is insufficient to generate a current in the load circuit of the detector.

SIDNEY Y. WHITE. 

